I'm making a "Time Stop", but it only works in Studio and not in-game, is there a way to fix this?

So a few things, you shouldn't be using KeyDown as it is deprecated, so I would recommend using the UserInputService. I also don't think you can get a player's keyboard inputs from the server.

A better way to set this up would be to put this code in a local script, and use a remote event

Create a RemoteEvent in Replicated Storage, name it "FreezeEvent" for now, then create a Local Script in StarterPack with the following code

local player = game:GetService("Players").LocalPlayer
local char = player.CharacterAdded:wait() -- Waits for the character

local UIS = game:GetService("UserInputService")
local RS = game:GetService("ReplicatedStorage")
local freeze = RS:WaitForChild("FreezeEvent") -- You can name this remote event whatever you want

UIS.InputBegan:connect(function(input,gpe) -- InputBegan event takes 2 parameters, the InputObject and GPE(Game Processed Event) 
	if not gpe then -- Makes sure you can't use this ability while typing.
		if input.KeyCode == Enum.KeyCode.Q and player.Name == "bearpro2" then -- You can change the controls to whatever
			freeze:FireServer() -- Calls the server via the RemoteEvent
		end
	end
end)

Then create a Script in ServerScriptService with the following code

local UIS = game:GetService("UserInputService")
local RS = game:GetService("ReplicatedStorage")
local freeze = RS:WaitForChild("FreezeEvent") -- You can name this remote event whatever you want.

-- Listens for the RemoteEvent to be called
freeze.OnServerEvent:connect(function(player) -- Remote events always return the player who sent the request.
	for i,v in pairs(workspace:GetChildren()) do
		if v:FindFirstChild("Humanoid") and v ~= player.Character  then
			if (v.HumanoidRootPart.Position - player.Character.HumanoidRootPart.Position).magnitude <= 200 then -- use HumanoidRootPart so it's compatible with both R6 and R15
				for i,child in pairs(v:GetChildren()) do
					-- BasePart includes both Parts and MeshParts
					if child:IsA("BasePart") then -- Loops through all the parts inside the model instead of looking for specific parts.
						child.Anchored = true -- Anchors it.
					end
				end
			end
		end
	end	
end)

lol remember to mark me as the answer if this helps you btw